A. Technical Field
The invention relates to a power supply apparatus of switching regulator type for a multi-frequency display system wherein plural horizontal synchronous pulse trains of different frequencies are used to support multi-display mode.
B. Prior Art and Problem
A switching regulator called as a switching mode power supply device has been used as a stabilized DC power supply for a CRT display device, a personal computer, a printer of a display workstation. The power supply device is mounted within a space in the CRT display device. Problems encountered in mounting the switching mode power supply device within the CRT display device are a jitter or a beating noise appeared on a CRT display surface due to an interference between a frequency of horizontal deflection pulses and a switching frequency of the switching regulator, and an unstability of a deflection magnetic field of the CRT display device caused by a leaked magnetic flux from a transformer of the power supply device. First approach for solving the problems was to magnetically separate the horizontal deflection circuit from the power supply by a shield. Second approach was to synchronize the horizontal deflection frequency with the switching frequency, as shown in "Switch mode power supply for CRT display," IBM Technical Disclosure Bulletin, Vol. 29, No. 9, February 1987, pp. 3921-3922, and "Switched power supply synchronized to display", IBM Technical Disclosure Bulletin, Vol. 29, No. 11, April 1987, pp. 4948-4949. In the former article, the switch mode power supply is normally run at a multiple of horizontal scan frequency. In the latter article, the switch mode power supply operates at halt the horizontal scan rate. Japanese patent application 63-294727 discloses a power supply device in which inputs of first and second switching regulators are commonly connected to DC bulk voltage output of a rectifying and smoothing circuit, and the second switching regulator operates at a switching frequency which is N times as large as or 1/N times as small as the switching frequency of the first switching regulator (N is integer larger than 1).
A multi-frequency type CRT display device has recently been marketed which has plural display modes of different horizontal synchronous frequencies, for example 31 KHz, 48 KHz, etc. to support various character display modes. In most of the multi-frequency type display devices, the horizontal synchronous frequencies and the switching frequency of the power supply device are independent from each other, and the shield for magnetically isolating both the circuits is used. In a recent display terminal device, the switching frequency of the power supply is designed to the horizontal synchronous frequency or its multiples. Actually, the multiple is used to realize a high efficiency and a reduced size of the power supply. In the case that the horizontal synchronous frequencies, e.g. 31 KHz and 48 KHz, are used, a power supply device is used which has the switching frequencies 93 KHz (=31 KHz.times.3) and 144 KHz (=48 KHz.times.3). That is, the power supply device must be designed to cover the wide range of 51 KHz (=144-93 KHz). Generally, typical circuit parameters such as core size of a switching transformer, switching speed range of a rectifying diode, capacitance and size of a smoothing capacitor, and design of heat reduction of circuit components are selected in accordance with the switching frequency or operating frequency of the switching regulator to design the switching mode power supply device.
In the design of the power supply device covering the wide frequency range 53 KHz, it was very difficult to select the above key design factors to optimum values all over the operational frequency range.